1. Industrial Field of the Invention
The present invention relates to a core for a coil device such as power transformers, choke coils used in switching power supply.
2. Prior Art
Generally, in constructing a power transformer for a switching power supply, two E-shaped segments are combined to form the core. A space is provided in the core forming the magnetic circuit, or the core cross-sectional area through which the magnetic circuit passes is portionately narrowed, so that magnetic flux leaks to the outer side of the core. This prevents magnetic saturation of the core even with increased current, up to a certain limit, due to load fluctuation.
FIG. 7 indicates a top view of a conventional power transformer core for a switching power supply that is composed of two E-shaped segments.
A projection 32 is provided at the end of a leg 31 in the center of an E-segment 30. This projection 32 contacts the flat end of the central leg 34 of another E-segment 33. Both outer legs of the E-segments 30 and 33 have flat ends, which are mutually in close contact. The legs 31 and 34 are inserted in the coil winding bobbin.
When a core is constructed in this manner, as the coil current increases, the projection 32, which has a narrowed cross-sectional area, is initially saturated. Since magnetic flux leaks to the perimeter of the projection 32, the overall core does not saturate magnetically until a large current is reached.
FIG. 8 is a characteristic indicating the relationship between a power transformer coil inductance L and current I. The solid line indicates the response when the projection 32 area is narrow.
When the current I is small, the inductance L increases. Even if the current I increases, since the core is not easily saturated, the inductance L exists up to a large current.
However, as indicated by the dotted line, if the projection 32 area is widened, although the inductance L increases when the current I is small, at a comparatively small current the overall core becomes magnetically saturated and inductance L ceases to exist.
The characteristic indicated by the solid line in FIG. 8 is sought in coil devices such as power transformers, choke coils used in switching power supply. Under the conventional method, to obtain such a characteristic, the cross-sectional area of the projection 32 needs to be narrowed.
However, when the cross-sectional area of the projection 32 is made narrow, the projection 32 is easily broken during core formation, and the core yield is reduced. Also in assembling a core with a narrow projecting 32, damage occurs easily and the core often needs to be rejected.